The inventive concepts described herein relate to a nonvolatile memory device and a method of configuring a neuromorphic system using the same.
A semiconductor memory device is typically volatile or nonvolatile. A volatile memory device supports fast read and write speeds, while it loses contents stored therein when a power to the volatile memory device is interrupted. The nonvolatile memory device retains contents stored therein although a power to the nonvolatile memory device is interrupted. Therefore, the nonvolatile memory device is used to store contents that must be retained regardless of whether a power is supplied to the nonvolatile memory device.
In general, a flash memory device is utilized as a high-integration and mass nonvolatile memory device. In addition, the high-integration and mass nonvolatile memory device is implemented by a ferroelectric random access memory (ferrolectric RAM or FRAM) using a ferroelectric capacitor, a magnetic random access memory (magnetic RAM or MRAM) using a Tunneling magneto-resistive (TMR) film, a phase change memory device using Chalcogenide alloys, a resistive random access memory (resistive RAM or RRAM) using a resistive material film as a data storage medium, etc.
A phase change memory cell utilizes a material that is switched between different structured states indicating different electrical read characteristics. For example, there are known memory devices fabricated using a Chalcogenide material (hereinafter, referred to as GST material) being Germanium-Antimony-Tellurium (GeSbTe). The GST material has an amorphous state showing a relatively high resistivity and a crystalline state showing a relatively low relativity. That is, data corresponding to the crystalline state or the amorphous state is stored in the phase change memory cell by heating the GST material. Heating conditions (e.g., level, duration, etc.) typically determine whether the GST material remains at the amorphous or crystalline state. A high resistivity indicates a written logical value of ‘1’, and a low resistivity indicates a written logical value of ‘0’. Such logical values are sensed by measuring a resistivity of the GST material.
A neuromorphic system means a system that mimics the manner in which a brain processes data by simulating a neuron of a human brain using various elements.
FIG. 1 is a diagram schematically illustrating a process in which a neuron receives an external signal and a process in which a neuron amplifies an input signal and transfers the input signal to an adjacent neuron. A synapse potential Vsyn being an electrical pulse form of the input signal is received at a dendrite of a pre-synapse neuron. At this time, if a sum of several synapse potentials Vsyn is higher than a threshold potential Vth, the neuron generates an action potential Vact and transfers the action potential Vact to a post-synapse neuron. This is referred to as neuron spiking.
FIG. 2 is a diagram showing voltages of pre-synapse and post-synapse neurons. FIG. 3 is a diagram showing a STDP (Spike-Timing Dependent Plasticity) algorithm.
STDP (Spike-Timing Dependent Plasticity) is a basic algorithm that adjusts the strength of connections between neurons, which is crucial for memory and learning. Neuron spiking generated through the above-described process is provided to a post-synapse neuron. The strength of connections between neurons that determines memory and learning of the human brain is determined by a time interval between an action potential of a pre-synapse neuron and an action potential of a post synapse neuron. If the time interval is negative, a long-term depression (LTD) phenomenon, where the strength of connections between neurons becomes weak, occurs. If the time interval is positive, a long-term potentiation (LTP) phenomenon, where the strength of connections between neurons becomes strong, occurs.
In general, the neuron spiking and STDP are simulated using a semiconductor element including a CMOS logic. In this case, since a lot of power is consumed and a wide area is required to implement a synapse element, it is typically difficult to highly integrate the synapse element.